If you want an actual good reason? Car damage. I have seen them roll away into someone else's car, and even a person. They can also block parking spots and it really sucks when people in the handicap section do it as those then roll in to other spots needed for accessibility. Thankfully my store has tons of cart returns so it isn't even remotely inconvenient. The employees stay on top of the rest, especially if the handicap section right by the doors gets gnarly. The only time I see some true cart carnage is at Walmart. Those parking lots are terrible.

I can't make decisions for you, but I will politely ask you to put the cart away the next time you are at the store. Please and thank you.